The present invention relates to a rotary body drive control apparatus used in an image forming apparatus such as a color electrophtographic copier and a color printer to control the drive of a rotary body such as a photoreceptor drum and a transfer drum.
For example, in a tandem-type color electrophotographic copier, a plurality of recording sections are sequentially arranged along a conveying path of a recording sheet that is conveyed by a transfer belt. Each recording section consists of a drum-shaped latent image carrying body, a developer for developing the latent image on the image carrying body with toner, a transfer device for transferring the toner image from the image carrying body to the recording sheet, and other devices. In the respective recording sections, toner images of different colors are transferred, i.e., superimposed onto the recording sheet being conveyed by the transfer belt. To correctly transfer the toner image onto the recording sheet at a predetermined position, it is necessary that the transfer belt be moved at a constant speed and the drum-shaped image carrying body be rotated at a constant angular velocity. To this end, the angular velocity of the image carrying body is detected and the rotation of a motor (rotational drive source) is controlled so that the angular velocity of the image carrying body is kept constant.
For example, Japanese Patent Application Unexamined Publication No. Sho. 63-75759 discloses a conventional rotary body drive control apparatus of the above type. This drive control apparatus for an image carrying body is used in an image forming apparatus that forms an image on a moving endless image carrying body, and has a step motor for driving the image carrying body through an image carrying body moving mechanism that is a reduction gear train having an integer-to-integer gear ratio, a memory for storing a one-rotation pulse generation pattern for canceling out a rotational variation in one rotation of the final stage gear of the gear train, and a means for detecting a home position of the final stage gear. While the image carrying body is moved, the step motor is driven with pulses being generated based on the stored pulse generation pattern.
In the drive control apparatus for the image carrying body disclosed in the above publication, the pulse generation pattern for canceling out the rotational variation of the image carrying body is stored in advance in the memory as a fixed pattern. The rotational variation of the image carrying body is prevented by driving the step motor using the pulses generated based on the fixed pulse generation pattern stored in the memory. However, the fixed pulse generation pattern cannot accommodate an unexpected change of the rotation states of the reduction gear train as would be caused by environmental variations such as a temperature variation, long-term use, etc. Therefore, the rotational variation of the image carrying body as would be caused by environmental variations or long-term use cannot be suppressed properly, and the residual rotational variation of the image carrying body will cause a color registration error, pitch unevenness (banding), etc.
To solve the above problem, there was proposed a rotation control method and apparatus in a multiple transfer apparatus (Japanese Patent Application Unexamined Publication No. Hei. 2-43574). According to this rotation control method in a multiple transfer apparatus in which a plurality of images are multiple-transferred onto a common transfer roller, information on the angular velocity variation of the transfer roller when a motor for driving the transfer roller is rotated at a constant angular velocity is stored in advance in a memory means. In the transfer operation, the angular velocity of the motor is adjusted based on the information on the angular velocity variation that is read from the memory means. Further, to accommodate the angular velocity variation of the transfer roller due to environmental variations or long-term use, an angular velocity signal from an encoder that is associated with the transfer roller is used. In this manner, the variation components are corrected. That is, the feedforward control and the feedback control are combined in this method to correctly register images of the respective colors onto a recording sheet.
According to the above rotation control method, the additional rotational variation of the image carrying body due to environmental variations or long-term use can be corrected as part of the variation components in adjusting the angular velocity of the motor based on the information on the angular velocity variation of the transfer roller that is read from the memory means and the correction information that is obtained on a realtime basis. Thus, this rotation control method can accommodate the rotational variation due to environmental variations, long-term use, etc.
However, the above conventional technique is still associated with the following problems. That is, in the above rotation control method, the information on the angular velocity variation itself of the transfer roller is stored in the memory means, and the angular velocity of the motor is directly adjusted based on the information read from the memory means. If it is intended to improve the accuracy of the rotation control of the transfer roller by greatly increasing the number of divided sections for the angular velocity detection, the variation component by the correction may gradually increase to become a vibration source with respect to a system from the motor to the rotation shaft of the transfer roller via gears. This may cause an oscillation or increase the amplitude of a vibration at the characteristic frequency of the system. Further, since in this case (that is, when it is intended to improve the accuracy of the rotation control of the transfer roller) it is necessary to use a highly accurate encoder to detect the angular velocity variation of the transfer roller, the cost increases as much.
Another publication Japanese Patent Application Unexamined Publication No. Sho. 58-66589 discloses a technique of stabilizing the rotation of an electrically driven motor by performing adaptive prediction type automatic control while detecting variations of the rotational phase and speed of the motor.
This technique can control the frequency itself that depends on the rotation cycle of the motor. However, for instance when the motor is used as a direct motor, that is, in the case of control in a relatively low frequency band, there is no gain for an angular velocity in a frequency band higher than that of the intended control. Therefore, there may occur a color registration error and banding.